Worldwide efforts are underway to improve the quality of video signal production, transmission, and reproduction because a great deal of commercial importance is being predicted for improved quality video systems. These efforts involve, at least in part, increasing the resolution with which images are converted into representative electrical signals by increasing the spatial and temporal sampling rates that are used to convert video images into electrical signals. This increase in resolution consequently means that more data about images must be produced, processed, and transmitted in a given interval.
Video images such as those images in the field of view of a television camera are scanned at a predetermined rate and converted into a series of electrical signals, each electrical signal representing a characteristic of a predetermined region of the image generally referred to as a picture element ("pel"), or pixel. A plurality of the pels taken together at a predetermined instant of time form what amounts to a still picture (i.e., a frame) representing the nature of the image at the predetermined instant of time. Increasing the quality of video signals produced in this manner involves, at least in part, the use of a larger number of smaller-size pels to represent a given image, and the production of a large number of images per unit of time.
As the number of pels for each video image and the rate at which images are produced increases, there is an increasing amount of video data which must be produced, transmitted, and received in a given interval. A number of data compression schemes have been proposed which attempt to transmit higher quality video images using the same numbers of bits and the same bit rates used for lower quality images. The Motion Picture Experts Group Phase 1 ("MPEG-1") standard provides a particular syntax and decoding process for one such scheme. This standard is set forth in International Standards Organization ("ISO") Committee Draft 11172-2, "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s", November 1991.
It may be desirable to obtain one or more lower resolution images from a single transmitted high-resolution video signal. For example, a video signal simultaneously transmitted to both high-definition television ("HDTV") and standard television receivers may have to provide images having a very high degree of resolution to the HDTV receivers, and images having a lesser degree of resolution to the standard receivers. Similarly, the degree of image resolution which need be obtained from a video signal displayed on a windowed computer screen must be varied with the size of a particular window in which it is displayed.
One known method of providing a video signal from which images of varying resolution may be derived is to simultaneously transmit a set of independent replicas of a video sequence; each replica being scaled for reproduction at a different level of resolution. This approach, referred to as "simulcasting", is simple, but requires an increased bandwidth to accommodate the transmission of multiple independent video images. A more bandwidth efficient alternative to simulcasting, is scalable video. Scalable video is a technique wherein a video signal is encoded and the resulting bit-sequence is partitioned so that a range of resolution levels may be derived from it depending upon the particular signal decoding scheme employed at a receiver.
Unfortunately, the encoding of scalable video is not provided for within the constraints of most video standards. A particular limitation of the MPEG-1 standard coding is its lack of provisions facilitating scalable video encoding.